High frequency heating system



Aug. 4, 1959 P. LAVAYSSIERE HIGH FREQUENCY HEATING SYSTEM Filed April 5, 1957 3 Sheets-Sheet l Aug. 4, 1959 P. LAVAYSSIERE HIGH FREQUENCY HEATING SYSTEM 3 Sheets-Sheet 2 Filed April 3, 1957 3 Sheets-Sheet a P. LAVAYSSIERE HIGH FREQUENCY HEATING SYSTEM Aug. 4, 1959 Filed April 3, 1957 United States Patent HIGH FREQUENCY HEATING SYSTEM Paul Lavayssiere, Vincennes, France, assignor to Etablissements T.R.M.-Technique, Radio, Mecanique, Neuilly-sur-Seine, France, a company of France This invention relates to high frequency generators for heating systems operating by dielectric loss effect. In

.such systems .a dielectric material to be heated is posi tioned between condenser electrodes across which a high frequency voltage is supplied. These systems have been applied with particular advantage for bonding and/or preheating plastic sheet materials.

Among the diificulties encountered in such systems is the fact that the final temperature of the material is difiicult to control and is changed by such factors as the network supply voltage and the temperature elevation of the electrodes. Moreover, when bonding thin plastic sheet material, sparking frequently occurs and the sparks burn through the sheet material to be bonded. A uniform end product is therefore difficult to obtain unless special precautions are taken.

The high voltage is usually applied to the material to be heated for a constant lapse of time, but this averts none of the difficulties just mentioned. It has also been suggested that the generator output be controlled in accordance with the variations in capacity between the electrodes. But this method is difiicult to carry out in practice owing to the very low magnitude of the controlling effect.

A more etlicient method would consist in measuring the D.-C. resistance of the material being heated, which resistance decreases with temperature, and cutting off the generator output when such resistance has dropped to a predetermined value. The resistances that have to be measured, however, are very high in value, and the resulting apparatus are therefore quite delicate to operate. More important, the resistance of the material being heated is proportional to the electrode surface area, so that the control device has to be recalibrated every time the electrodes are replaced.

It is an object of this invention to provide control or regulating apparatus applicable to high frequency generators for dielectric heating systems, whereby the end temperature in the material being heated will be independent of the depth of the material, the electrode area, the supply voltage applied, and other operating parameters. Another object is to prevent arcing across the electrodes. Another object is to provide such apparatus especially suitable for bonding or laminating thin plastic sheet material.

According to the invention, means are provided for measuring at all times the loss angle in the material being heated and means for automatically controlling the generator output in response to the changes in said loss angle.

This can be accomplished in various ways. Thus in intermittently operating machines, such as press bonding machines for plastic sheet material, the generator is cut off when the loss angle has reached a predetermined value. In continuously operating machines, for example roller'bonding machines, a substantially constant generator output may be retained and the rate of feed of the plastic sheets may be controlled so as to maintain a constant loss angle, or alternatively a uniform feed rate may be used and the generator output may be controlled to maintain a constant loss angle.

The loss angle of the material is approximately equal to the reciprocal of the voltage across the condenser defined by the two electrodes and the heated material as the dielectric. It depends only on the nature and temperature of the heated material but not on the depth of the material or on the electrode area. Using the loss angle as a controlling factor, therefore, makes it possible to place a tight control on the end temperature regardless of any disturbances, and no readjustment is necessary until the work material is changed.

Further, the occurrence of a spark instantly and greatly modifies the loss angle and can be used immediately to cut off the generator and thus prevent damage which would otherwise occur.

Nevertheless surface sparking can still occur and burn the surface portions of the work material without substantially modifying its loss angle and hence without operating the cut-off device just mentioned. Since however the sparks are accompanied by a rapid variation in the generator output current, means may be further provided according to the invention for immediately cutting off the high frequency output whenever such a variation occurs.

The invention will be fully understood from the ensuing description and accompanying circuit diagrams given by way of non restrictive examples:

Fig. 1 illustrates a general block diagram of a system according to the invention;

Fig. 2 is a typical curve illustrating loss angle variation with temperature;

Fig. 3 illustrates the system of the invention as applied to loss angle measurement with an auxiliary generator;

Fig. 4 illustrates the time variations of a particular voltage value in the circuit of Fig. 3;

Fig. 5 shows a preferred embodiment of the invention which does not use an auxiliary generator; and

Fig. 6 shows means for preventing surface sparking efiects apt to damage the work.

The same reference numerals serve to designate simi lar components throughout the drawings.

As shown in Fig. 1, 1 is a high frequency generator serving to apply a suitable voltage across the electrodes of a condenser 3 between which the material to be heated is placed. The losses in the material may be represented by .a resistance 4 in parallel (or in series) with the condenser 3. According to the invention there are pro vided means 5 for measuring the current flowing through condenser 3 and means 6 measuring the voltage across the condenser. A phase comparison device 2 measures the phase displacement between the said current and voltage, which is the loss angle in the material being heated. The output of the phase comparison device 2 is used to operate the controls 8 of the generator 1 so that the said phase displacement is maintained substantially constant.

The loss angle in the heated material increases with temperature as indicated in Fig. 2 and hence a definite temperature value corresponds to each value of the loss angle.

The loss angle in the material may be measured at the same frequency as that of the power generator or it may be measured at some other frequency. In the latter case an auxiliary generator 7 has to be provided and the means 2, 5 and 6 are tuned to the frequency of the auxiliary generator.

Fig. 3 shows one example of a circuit according to the invention using a low-power auxiliary generator for measuring the loss angle in the work material. The power generator 1 comprises a triode 10 connected in an oscillator circuit. This includes a tank circuit 11 connected to the anode of tube and inductively coupled to an inductance 14 connected between the control grid of the tube and ground through a bias resistance 15 and capacitance 16 in parallel. The plate of the tube is connected by way of a choke inductance 13 to a rectifier comprising a pair of diodes 19 and 20 and a power transformer 21 having its primary side connected across the power network 22 by way of a pair of switches 23 and 24.

The generator output is tapped from a point of the inductance of tank circuit 11 by way of a condenser '17 and is applied through a resonant circuit 18 across the electrodes of the heating condenser 3. The parallel resistance 4 represents the impedance in the material being heated.

A low-power auxiliary generator 7 comprising a triode 43 and tuned oscillatory circuit 44 is supplied with plate voltage from network 22 by way of rectifiers 42 and transformer 41, and the output of the auxiliary generator is applied across the heating electrodes 3. For this purpose a winding of the oscillator transformer 44 is connected in series with the condenser 3, a choke coil 48 and a network comprising resistance 45 and inductance 46 and series condenser 47 in parallel.

The voltage appearing across output resistance 45 is applied to the primary of a transformer 38 and the voltage appearing across condenser 3 is applied across the primary of a transformer 39 by way of a choke coil 40. The secondary of transformer 39 has a midpoint connected to one end of secondary of a transformer 38 and its ends connected to the anodes of diodes 32 and 33 the cathodes of which are respectively connected to the resistances 34 and 35 and capacities 36 and 37, the common junctions of which are connected to the other end of the secondary of transformer 38.

Diode 33 has its cathode grounded and diode 32 has its cathode connected to the grid of a thyratron tube 30 by way of a D.-C. source 31 having its negative terminal connected to the thyratron grid. The cathode of the thyratron is grounded and its plate is connected through a switch 29 to the cathode of a rectifier valve 27 including a pair of anodes connected across the secondary of a transformer 26 the primary of which is connected across the power supply. The midpoint of the secondary of transformer 26 is connected to a condenser 28 having its other terminal connected to the cathode of the valve 27. The midpoint is moreover connected to the grid bias resistor 15 through a relay winding controlling the power cutoff switch 24.

In Fig. 3 the switches are shown in their normal positions and the frequency of auxiliary generator 43 is assumed to be lower than the frequency of power generator it This is by no means essential since the frequency relationship could easily be reversed and the necessary changes in the circuit would be readily accomplished by those familiar with the art.

The operation of the system will be explained with reference to Fig. 4 in which the grid voltage V of the thyratron is plotted as a function time or the temperature of the work, assumed to be increasing with time.

To start a heating operation both switches 23 and 29 are simultaneously closed. This applies power across the electrodes and prepares thyratron 30 for operation. The thyratron does not fire at this time however since its grid is held at a negative cut-01f bias. The current from auxiliary generator 7 flows through the two impedances respectively provided by resistance 4 with condenser 3, and by resistance 45, inductance 46 and condenser 47. The phase displacement through the first of these impedances is very nearly equal to the loss angle in the work material, and the phase displacement through the second impedance network may be selected at any suitable value. Thus, resistance 45 may be so adjusted for example that both phase displacements are in quadrature relation when the loss angle in the work material has attained a predetermined value, corresponding to the desired end temperature. The phase comparison circuit within the box in broken outline designated 2 operates in a well-known manner to supply a zero output when both input voltages thereto are in phase quadrature, regardless of the amplitude value of each input.

The inputs to the phase comparison circuit 2 are not in phase quadrature at the beginning of the heating operation but they approach such quadrature relationship as the temperature of the heated material rises and the loss angle of the material changes correspondingly. The phase comparison circuit 2 is so connected as to deliver a negative output at the start of the operation, said output approaching zero as the material approaches the desired end temperature (Fig. 4). The output voltage from circuit 2 is applied to the control grid of the thyratron, and the negative bias voltage of the source 31 is selected equal to the firing voltage V so that the thyratron will fire when the output of circuit 2 is null, that is to say when the desired end temperature has been attained.

When the thyratron fires the rectified voltage across condenser 28 is applied across the resistance 15, thereby biasing the power oscillator grid beyond cutoff and blocking the operation of the power oscillator 1. At the same time relay 25 is energized and its contacts 24 open, removing the plate voltage supply from the oscillator. The switches 23 and 29 may then be opened to condition the system for a fresh operating cycle.

A few precautionary measures should be taken in connection with the circuit just described to prevent interference between the outputs of oscillation generators 1 and 7. Thus oscillatory circuit 18 is tuned to the frequency of oscillator 7 so that the signal from said circuit will not become lost in the tuned circuit 11. Similarly the choke inductances 40 and 48 prevent the output from oscillator 1 from interfering with the operation of the phase discriminator 2, an additional precautionary measure is to select the values of inductance 46 and capacity 47 so as to obtain series resonance for the output freqency of oscillator 1.

Fig. 5 illustrates a preferred form of the circuit of the invention wherein the loss angle of the work material is measured directly at the operating frequency without the aid of an auxiliary generator. This results in considerably simplifying the equipment involved.

The power generator 1 and phase comparison device 2 are similar to the corresponding components described with reference to Fig. 3. This applies also to the generator cutofi circuit 8 comprising thyratron 30, associated rectifier valve 27 and relay 35. The phase comparison device 2 is tuned to the output frequency of generator 1.

The losses in the work material are here represented for greater convenience by a resistance 4 in series with the heating condenser 3. In parallel with the condenser is a series network comprising condenser 60 and resistance 61 condenser 60 being substantially smaller in capacity value than condenser 3 and the phase angle of the impedance comprised of condenser 60 and resistance 61 being substantially the same as the phase angle provided by condenser 3 and resistance 4 at the desired end temperature, in other words as the loss angle in the material being heated.

The primary of transformer 39 in the phase comparison circuit is supplied with the voltage appearing across the resistance 61 and the primary of transformer 38 is connected to be traversed by the current flowing through the heating electrodes. Therefore, the voltage across the secondary of transformer 39 is in phase with the voltage across resistance 61. The voltage across the secondary of transformer 38 is in phase quadrature with the current flowing through condenser 3 (since the condenser 60 has a much lower value) and hence with the current through resistance 4. The phase comparison circuit will consequently deliver a zero output when the currents through resistance 4 and resistance 61 are in phase, i.e. when the loss angle in the work equals the phase angle of condenser 60 and resistance 61.

The operation of this system is generally the same as that of Fig. 3. The thyratron grid voltage varies in the manner indicated in Fig. 4 and the thyratron fires when the phase comparison device output applied to it is zero, which occurs when .the desired end temperature has been attained. 'The generator is then cut off: in

the same manner as described for the Fig. 3 circuit.

The advantages of the system over conventional systems will be readily apparent. The main advantage is that the loss angle of the material being heated is not dependent on the nature of the material nor on the depth of material nor on the electrode area of the condenser 3. Since the phase discriminator or comparison device is used for detecting the quadrature relationship between the two signals and when such relationship is present the output of the device becomes zero, it is seen that regardless of the respective amplitudes of said signals, a constant adjustment of the control device may be retained provided only the same material is being heated.

Moreover, all high-frequency generator control systems of electronic character are somewhat delicate in that the are easily disturbed by the very high-power, highfrequency output of the generator. This difliculty is completely averted in the system of Fig. 5 since the control system operates on the same frequency as the poWer generator.

Furthermore, the occurrence of a spark through the work material has the effect of substantially converting the condenser 3 into a resistance. This results in a phase displacement which immediately arrests the operation of the generator without damaging the electrodes or the work.

Occasionally however surface sparks may occur which might be liable to burn the surface portions of the heated material without burning through the material and hence without causing the above described protective action to take place. To avert such a condition, the invention contemplates provision of an additional device as shown in Fig. 6.

A transformer 70 has its primary connected in series in the D.-C. supply circuit for the power generator triode. The primary of the transformer may, instead, be connected in series with the choke inductance 13, but it is more convenient for insulation considerations to connect it in series with the cathode as shown. Since high power tubes of this kind generally use direct heated cathodes, the primary winding comprises, as shown, a twin conductor and each conductor is connected to a respective end of the filament and has its opposite end connected to a corresponding end of the secondary of heating transformer 71 and a de-coupling condenser 72. The secondary of transformer 70 has one end connected through ground to the cathode of thyratron 30 and its other end connected through capacity 73 to the grid of the thyratron as indicated by connection 50 in Figs. 6 and 3. The transformer 70 comprises a small number of turns so as not to pass the lower frequencies.

When a surface spark occurs on the electrodes the D.-C. current flow through triode 10 is subjected to rapid rate changes or fluctuations which are transmitted by transformer 70 to the grid of thyratron 3t and the thyratron fires and cuts off the generator before the sparks have had time to burn through the work.

It will be understood that many modifications may be made in the illustrated circuits by anyone familiar with the art. Thus, in Fig. 5, the ungrounded electrode of condenser 3 in some cases may only ,be accessible by way of a conductor having a non-negligible self inductance value. A corresponding inductance could then be inserted in series with condenser 60.

operation of the generator in response to Zero output from the phase comparator, may be replaced by any suitable equivalent means. The phase comparison device itself may be modified in many different ways.

' Although specific reference was made in the descrip tion to'machines for bonding or laminating plastic sheet materials, of'the intermittently operated or press type, it will be apparent that the invention is by no means restricted'to this specific use.

What I claim is:

1. In a heating system for dielectric material, a pair of electrodes adapted to receive said material therebetween to define a condenser therewith, an A.-C. power generator applying an A.-C. voltage across said electrodes to heat the material through dielectric loss effect, the phase displacement between the voltage across the electrodes and the current through the material being a measure of the temperature in said material, and means sensing said phase displacement and connected to said generator to cut off said generator with said phase displacement at a predetermined value.

2. In a heating system for dielectric material, a pair of electrodes receiving said material therebetween, a generator applying an A.-C. voltage across the electrodes to heat the material through dielectric loss effect, the phase displacement between said voltage and the current through the material being a measure of the temperature in the material, and means sensing said phase displacement and connected to said generator to cut off said generator with said phase displacement at a predetermined value.

3. In a heating system for dielectric material, a pair of electrodes receiving said material therebetween, a generator applying an A.-C. voltage across the electrodes to heat the material, the phase angle between said voltage and the current through the material being a function of the temperature in the material, means sensing said phase angle, and means connected to the generator and the sensing means to cut off the generator output with said phase angle at a value corresponding to a preselected temperature in the material.

4. In a heating system for dielectric material, a pair of electrodes receiving said material therebetween, a generator applying an A.-C. voltage across the electrodes to heat the material, the phase angle between said voltage and the current through the material being a fume tron of the temperature in the material, phase comparison means having two inputs and an output, additional means applying to said inputs respective signals corresponding to said voltage and said current flow whereby said output delivers a signal corresponding to said phase angle, and means applying said output signal to the generator to cglt off said generator with said signal at a predetermined v ue.

5. The system claimed in claim 4 wherein the last said means cuts off said generator when said output signal reaches a null value.

6. In the system claimed in claim 4, means in said comparison means for introducing a predetermined phase dlsplacement between said input signals, whereby said output signal is zero when the phase angle has attained a predetermined value corresponding to a preselected end temperature in the material, and means cutting off the operation of the generator when said output signal is zero.

7. The system claimed in claim 4, wherein the said additional means applies input signals to said comparison means at the same frequency as the A.-C. voltage of the power generator.

8. The system claimed in claim 4, comprising an auxiliary generator for producing said input signals at tion of the temperature in said material, means sensing .said phase angle, means connected to the generator and the sensing means to control the output of the generator so as to maintain said phase angle at a substantially constant value, means sensing rapid variations in said current, and means connected to the generator and to said further sensing means for cutting ofi the operation of the generator in response to such variations.

References Cited in the file of this patent UNITED STATES PATENTS 2,498,760 Kreithen Feb. 28, 1950 2,763,758 Kohler Sept. 18, 1956 2,785,264 Gillespie et al. Mar. 12, 1957 2,786,926 Rothstein et a1 Mar. 26, 1957 

